Method and apparatus for reducing emissions from breather lines of storage tanks

ABSTRACT

A method and apparatus is disclosed for separating a vapor-gas mixture formed above a liquid in a tank and which mixture is exhausted from the tank through an exhaust line in order to relieve overpressure in the tank. The mixture is passed from the tank through a membrane unit having a separating membrane and underpressure is generated at least intermittently, on a permeate side of the membrane unit, by a pump. This produces on the permeate side of the membrane permeate enriched with vapor and on the retentate side of the membrane retentate deleted in vapor. The permeate is returned to the tank and the retentate is exhausted to the atmosphere via the exhaust line.

BACKGROUND OF THE INVENTION

This invention relates to a method and apparatus for separating avapor-gas mixture formed above a liquid in a tank, the mixture beingexhausted from the tank via an exhaust line in order to relieveoverpressure. In particular, the invention relates to a method andapparatus for separating a mixture of hydrocarbon vapors and gases whichis to be exhausted from, e.g., an underground tank for liquidhydrocarbons, particularly motor fuels, for the purpose of relievingoverpressure in the tank by exhausting material through the breatherline of the tank.

In fuel filling stations, the fuel is generally stored in undergroundtanks which communicate with the atmosphere via at least one ventilationmeans, a so-called breather line. The objective is to avoid developmentof excessive overpressure (excessive positive gage pressure) orexcessive underpressure (excessive negative gage pressure) in the tank.Particularly when fuel is being added to or removed from the storagetank, the free volume above the liquid level changes, and, in order toequalize pressure, vapor-gas mixture from the free space is exhausted tothe atmosphere via the breather line, or, alternatively, air passes fromthe atmosphere into the tank via said breather line. The pressure in thetank can also vary due to temperature fluctuations in the tank orevaporation of liquid or fluctuations in ambient air pressure outsidethe tank. Aspiration of air from the surroundings to adjustunderpressure in the tank does not present particular problems withregard to, e.g., environmental contamination. Exhausting of vapor-airmixture, rich in fuel vapor, to adjust overpressure in the tank, can bean environmental burden, however, and can be injurious to the health ofpersons in the area.

So-called active recycling systems are known, for reducing emissions ofa vapor-gas mixture when filling the tank of a motor vehicle with fuelfrom a storage tank. These employ a pump to aspirate the vapor-gasmixture which is developed during such a filling operation, and toreturn said mixture to the storage tank of the filling station. As arule, such systems are operated in a manner such that the volume of therecycled vapor-gas mixture equals the volume of the fuel withdrawn. Inorder to increase the effectiveness of such recycling systems, it isadvantageous for the volume of the recycled vapor-gas mixture to be atleast equal to the volume of the fuel withdrawn, and preferably greaterthan that volume. Consequently, where a greater volume of recycle isintroduced to the tank than the volume of the fuel withdrawn, thepressure in the storage tank can actually be increased; thisoverpressure is then relieved by exhausting some of the vapor-gasmixture above the liquid into the atmosphere via the breather line. Thefilter element (if any) present in the breather line, under the priorart, tends to be inadequate, resulting in emission of a certain amountof fuel vapor, contained in said vapor-gas (vapor-air) mixture. Thus,while no doubt the emissions of toxic substances can be reduced in theimmediate vicinity of the filling station pump apparatus (i.e. thedelivery nozzle), a certain amount of fuel vapor will be emittedelsewhere at the filling station, because of the necessary pressureequalization, thereby reducing the effectiveness of the emission controlsystem.

SUMMARY OF THE INVENTION

The underlying problem of the present invention is to devise a methodand apparatus for reducing harmful emissions in the region of thebreather line of a storage tank for liquids, particularly liquids usedas motor vehicle fuels.

This problem is solved by a method according to the invention, in thatthe vapor-gas mixture is passed from the tank through at least onemembrane unit having at least one separating membrane, wherein anunderpressure is generated at least intermittently, on the permeate sideof said membrane unit, by at least one pump provided therefor, andwherein a permeate enriched with vapor of the liquid in the tank isreturned to the tank; and in that the retentate depleted in vapor ofsaid liquid is exhausted via the exhaust line. The inventive method canbe carried out with simple means. For example, semipermeable gasseparation membranes can be employed which are selectively permeable tovapors. In combination with the applied underpressure, the result can bethat a very high proportion of the vapors in the vapor-gas mixturepassed through the membrane unit is removed. The mixture ultimatelyexhausted through the exhaust line will thus have minimal, if any,adverse environmental impact.

According to an advantageous refinement of the invention, the pump isactuated when a prescribed overpressure is exceeded in the tank. Thissaves energy, in that the pump is only in operation when vapor-gasmixture is being exhausted. Also, with this mode of operation theexhaust line including the membrane unit can be used for admitting airto the tank to adjust for underpressure, without the risk of themembrane being soiled by particles of fouling material which may bepresent in the ambient air, because the pump will be turned off when theair is being admitted.

In another refinement of the invention, the exhaust line can be openedor shut by at least one overpressure valve, and the pump is actuateddepending on the position of the valve, the pump being turned on whenthe valve is open and turned off when the valve is closed. Thisarrangement avoids escape of liquid to the atmosphere via diffusion fromthe liquid to the vapor-gas mixture prevailing over the liquid and fromthe vapor-gas mixture to the atmosphere, which can occur even with thepressures equalized. Very simple means can be used to accomplish thismode of operation. Only one switch is needed, and this may be amechanical switch, actuatable by the valve body, e.g. the ball of a ballvalve, directly causing the pump to be switched on.

In another refinement of the invention, the pump for producing theunderpressure on the permeate side of the separating membrane isactuated depending on the position of a valve (or valves) whichcontrol(s) delivery of liquid from the tank and/or addition of liquid tothe tank, the pump being turned on when the valve is open. Thisarrangement provides reliable recovery of vapor from the vapor-gasmixture which is exhausted, at times when the fill state of the tank isbeing actively influenced. The method can be particularly advantageouslyemployed in conjunction with a delivery valve which is part of adelivery apparatus which has a so-called active recycling system of thetype described above, for vapor-gas mixtures which arise in transferringliquid, e.g. when fueling a vehicle from the tank. With such acombination, a certain volume of vapor-gas mixture is aspirated at thedelivery pump nozzle, which volume is generally greater than the volumeof liquid fuel withdrawn from the tank. During the process; pressure isequalized via the breather line, generally without any appreciableintervening pressure increase because the withdrawal (through the fueldelivery apparatus) is occurring at a low rate and the vapor-gas mixturewhich is being returned via the "active recycling system" is beingreturned at a low rate. In particular, if an overpressure switch is usedto control the pump there is a risk that under conditions of slowexhaust of the vapor-gas mixture the switch will not be triggeredbecause the pressure will not rise, and consequently all of the vapor inthe vapor-gas mixture being exhausted will be exhausted to theatmosphere, with none being removed via the separation unit.

An inventive apparatus is also provided according to the invention forseparating a vapor-gas mixture formed above a liquid in a tank, whichmixture is exhausted from the tank via an exhaust line in order torelieve overpressure. The apparatus is devised in particular forcarrying out the inventive method. It is comprised of at least onemembrane unit having at least one separating membrane. The inlet forfeeding the vapor-gas mixture to the membrane unit is connected to thetank, the outlet for the retentate from the membrane unit is connectedto the exhaust line, the outlet for the permeate from the membrane unit,which permeate is enriched in the vapor of the said liquid, is connectedto a pump which produces an underpressure on the permeate side of themembrane, and said pump returns the permeate to the tank.

An overpressure switch may be provided, the pressure sensor of which isconnected to the interior of the tank, wherein said switch cooperateswith the pump which produces an underpressure, such that said pump isactuated when the pressure in the tank exceeds a prescribed value.

In a refinement of the invention, at least one throttle element isprovided in the exhaust line, such that excess vapor-gas mixture presentin the container cannot be exhausted through the exhaust line at anyappreciable rate without developing an overpressure capable oftriggering the overpressure switch.

It may be advantageous for the throttle to be in the form of aperforated plate, which may be installed in simple fashion in theexhaust line of the tank, which line is typically in the form of one ormore tubular conduits.

According to another refinement of the invention, the exhaust line hasat least one overpressure valve whereby the exhaust line can be closedoff, and at least one control device comprised of a control switch whichcooperates with the overpressure valve and the underpressure-producingpump, so that when said valve is open the pump is operating and whensaid valve is shut the pump is shut off. This allows the apparatus forseparating the vapor-gas mixture being exhausted to be used as anadjunct to a tank for storing liquids (particularly fuels) where thetank has means of completely closing the system.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described in more detail hereinbelow withreference to the accompanying drawings.

FIG. 1 is a schematic view of a filling station installation providedwith an apparatus for carrying out the inventive method; and

FIG. 2 is a schematic view of a filling station installation providedwith an apparatus for carrying out an alternative embodiment of theinventive method.

DESCRIPTION OF PREFERRED EMBODIMENTS

The exemplary embodiments of the inventive apparatus illustrated inFIGS. 1 and 2 both have essentially the same basic configuration, anddiffer principally in the means of controlling a vacuum pump forproducing an underpressure on the permeate side of a separatingmembrane. Accordingly, corresponding components and elements are labeledwith the same reference numerals in the two Figures.

The filling station shown schematically in FIG. 1 has a storage tank 2for liquid fuel 3, connected to a vehicle-fueling delivery apparatus 4supplied therefrom. A ventilation apparatus (breather system) 5 isprovided which enables equalization of pressure between the tank 2 andthe atmosphere 6. Filling means 7 are also provided through which thefuel can be introduced into the tank 2.

The vehicle-fueling apparatus 4 is comprised of a delivery pumpapparatus 8 which delivers liquid fuel to a vehicle 10 via a flexiblehose 11 which leads to the vehicle-fueling delivery nozzle 9. Thefueling apparatus 4 also has an active recycling system 12 (not shown indetail) for returning to the tank certain volumes of vapor-gas mixturewhich are evolved in the course of the vehicle fueling. Recyclingsystems of this type (12) are generally known and will not be describedin more detail here.

The ventilation apparatus 5 has a membrane apparatus 29 which employs aseparating membrane 13. The inlet to the retentate side 14 of themembrane 13 i.e. the unit 29 is connected to the tank 2 via a line 15,and the outlet of the retentate side is connected to the exhaust line 16which eventually opens out to the atmosphere 6. The line 15 is open inthe upper region 17 of the tank 2 above the liquid level 18 in the tank.Thus, vapor-gas mixture (comprising fuel vapor and air) from the tank 2which mixture is to be vented can be exhausted to the atmosphere 6 onlyvia line 15, membrane unit 29, and exhaust line 16.

The permeate side 19 of the membrane unit 29 is connected to theunderpressure side of a vacuum pump 20 which pump communicates with thetank 2. The pump 20 when in operation applies an underpressure to thepermeate side 19 of the membrane unit 29, to promote permeation of fuelvapor through membrane 13 from the vapor-air mixture flowing through themembrane unit. Membrane 13 is advantageously a semi-permeable gasseparation membrane. The permeate thereby enriched in fuel vapor can bereturned to tank 2 by means of vacuum pump 20.

In the exemplary embodiment according to FIG. 1 an overpressure switchsystem 21 is provided which is comprised of a pressure sensor 22disposed in the upper region 17 of tank 2, and a switch 23 which, incoordination with sensor 22, controls the vacuum pump 20. The switchsystem 21 is chosen and adjusted such that when the pressure in tank 2exceeds a predetermined value the switch 23 is actuated, starting thevacuum pump 20. The vapor-air mixture present under conditions ofoverpressure in the tank 2 is subjected to exhausting to relieve theoverpressure. As said mixture flows through the membrane unit 29, vaporis separated out from it with the aid of the underpressure applied tothe membrane 13. The retentate eventually exhausted through the exhaustline 16 is thus largely free of fuel vapor. The exhaust line 16 has athrottle element 24 in the form of a perforated plate 25, to developsufficient pressure in the tank 2 to trigger the overpressure switch 21.

In the embodiment illustrated in FIG. 2 the exhaust line 16 has anoverpressure valve 26 which opens to allow exhaust of vapor-gas mixture.An underpressure valve 27 is also provided to allow aspiration ofambient air when an underpressure is present in the tank 2. overpressurevalve 26 is associated with a control switch 28 which cooperates withthe vacuum pump 20. The arrangement is chosen and adjusted such that thecontrol switch 28 is actuated when valve 26 is open, i.e. when vapor-gasmixture is being exhausted, whereby switch 28 turns on the pump 20. Thisprovides a reliable system for carrying out the separation process inthe membrane unit 29 when vapor-gas mixture is to be exhausted.

It can be readily appreciated that the embodiments illustrated in thedrawings facilitate reliable reduction of emissions of vapor in theregion of the breather line of a tank of a filling station. Means mayalso be provided to enable the pump 20 to be switched on in coordinationwith the actuation of the delivery valve of the fueling apparatus 4 orin coordination with the filling valve of the filling system 7. Inparticular, when an active recycling system 12 is employed wherebyvapor-gas mixture which is developed in the course of fueling a vehicleis returned to the tank 2, means can be provided whereby said returnmixture undergoes separation in the membrane unit 29 and the retentatetherefrom is exhausted through exhaust line 16 without any (or with onlyminor) pressure buildup inside the tank 2.

What is claimed is:
 1. A method for separating a vapor-gas mixtureformed above a liquid in a tank, which mixture is exhausted from thetank via an exhaust line in order to relieve overpressure in the tank,comprising passing the vapor-gas mixture from the tank through at leastone membrane unit having at least one separating membrane, generating anunderpressure at least intermittently, on a permeate side of saidmembrane unit, by at least one pump to provide on the permeate side ofthe membrane permeate enriched with vapor of the liquid, returning saidpermeate to the tank to provide on a retentate side of the membraneretentate depleted in vapor of said liquid, exhausting the retentate tothe atmosphere via the exhaust line, and opening and shutting theexhaust line by at least one overpressure valve and actuating the pumpdepending on the position of said valve, the pump being turned on whenthe valve is open and turned off when the valve is closed.
 2. A methodfor separating a vapor-gas mixture formed above a liquid in a tank,which mixture is exhausted from the tank via an exhaust line in order torelieve overpressure in the tank, comprising passing the vapor-gasmixture from the tank through at least one membrane unit having at leastone separating membrane, generating an underpressure at leastintermittently, on a permeate side of said membrane unit, by at leastone pump to provide on the permeate side of the membrane permeateenriched with vapor of the liquid, returning said permeate to the tankto provide on a retentate side of the membrane retentate depleted invapor of said liquid, exhausting the retentate to the atmosphere via theexhaust line, and actuating the pump depending on the position of avalve which controls delivery of liquid from the tank and addition ofliquid to the tank, the pump being turned on when the valve is open. 3.An apparatus for separating a vapor-gas mixture formed above a liquid ina tank, comprising an exhaust line for exhausting the vapor-gas mixturefrom the tank in order to relieve overpressure, at least one membraneunit having at least one separating membrane, an inlet to the membraneunit for feeding the vapor-gas mixture to the membrane unit from thetank, a first outlet from a retentate side of the membrane unitconnected to the exhaust line for delivering retentate depleted in vaporto the exhaust line, a second outlet from a permeate side of themembrane unit for permeate enriched in vapor and a pump connected to thesecond outlet to provide an underpressure on the permeate side of themembrane unit and return the permeate to the tank via a return line,wherein the exhaust line includes at least one overpressure valve and acontrol switch is provided which cooperates with the overpressure valveand the pump for operating the pump when said valve is open and shuttingoff the pump when said valve is shut.
 4. Process for separating amixture of liquid vapor and gas, which is produced above a liquid in atank and which is conveyed from the tank through at least one membranedevice with at least one separating membrane, on whose permeate side anegative pressure is produced by means of at least one pump device, anda permeate, enriched with the liquid vapor, is returned into the tank,comprising that to compensate for the overpressure the mixture of liquidvapor and gas is discharged in such a manner through a pipe from thetank that a retentate, largely free of liquid vapor, is dischargedthrough an outlet pipe, whereby the pump device is actuated as afunction of the overpressure prevailing in the tank as soon as aspecific overpressure is exceeded, wherein the outlet pipe can be closedby at least one overpressure valve and the pump device is actuated whenthe overpressure valve is open and deactivated when the overpressurevalve is closed as a function of the position of the overpressure valve.5. Process as claimed in claim 4, wherein the pump device is actuatedwhen the overpressure valve is open as a function of the position of afilling valve of the tank for the liquid.
 6. Device for separating amixture of liquid vapor and gas, which is produced above a liquid in atank and which can be discharged from the tank through a pipe andsubsequently through an outlet pipe in order to compensate for anoverpressure, comprising at least one membrane device with at least oneseparating member, whose feed side for the mixture of liquid vapor andgas is connected to the tank and whose retentate side is connected tothe outlet pipe, whereby in order to return a permeate, enriched withthe liquid vapor, a permeate side is connected to the tank by means of apump device in order to generate an underpressure on the permeate sideof the separating member and further comprising an overpressuretransducer, whose pressure sensor is connected to the interior of thetank and which interacts in such a manner with the pump to generate anegative pressure that, when a specific overpressure is exceeded, thepump is actuated, wherein the outlet pipe has at least one overpressurevalve, and further comprising at least one controller, which has acontrol switch and which interacts in such a manner with theoverpressure valve and the pump to generate a negative pressure so thatthe pump is actuated when the overpressure valve is open and isdeactuated when the overpressure valve is closed.